1. Field of the Invention
This invention relates to a circuit configuration method for a two-wire to four-wire switching exchanger which separates transmission/receiving signals of communication equipment connected to two-wire communication line such as main wire or PBX extension into four-wire transmission/receiving signals.
2. Description of the Prior Art
FIG. 2, for example, shows a block diagram illustrating a configuration of a conventional two-wire to four-wire switching exchanger which is described in the "MOTOROLA SEMICONDUCTOR TECHNICAL DATA DSP56200 Advance Information" (MOTOROLA INC.,1988). Referring now to FIG. 2, reference numeral 1 is a hybrid circuit, numeral 2 is a balancing network, numeral 3 a receiving amplifier, numeral 4 a transmission amplifier, numeral 5 A/D converter, numeral 6 D/A converter, numeral 7 a signal adding machine and numeral 8 a digital filter respectively.
Then, the operation of the conventional two-wire to four-wire switching exchanger will be described below. First, a receiving signal from a two-wire communication line is inputted from the hybrid circuit 1 and passes through the receiving amplifier 3, the A/D converter 5 and the signal adding machine 7 to become a four-wire receiving signal.
On the other hand, a four-wire transmission signal passes through the D/A converter 6 and the transmission amplifier 4 and sent out through the hybrid circuit 1 to the two-wire communication line. And at the same time, the transmission signal reaching the hybrid circuit 1 is not always outputted to the two-wire communication line, but a part thereof flows from the hybrid circuit 1 to the receiving amplifier 3 and follows the same route as the receiving signal to appear on the four-wire receiving output as an echo signal. The volume of this echo signal is in proportion to a discrepancy in impedance balance between the two-wire communication line and the balancing network 2.
Then, the four-wire transmission signal is processed to form an inverted signal at a size corresponding to the volume of said echo signal by means of the digital filter 8. When such an inverted signal is added to the signal outputted from the A/D converter 5 by the signal adding machine 7, the signal outputted from the adding machine 7 becomes the four-wire receiving signal leaving the two-wire receiving signal by cancelling only the echo signal.
In such a conventional two-wire to four-wire switching exchanger as constructed above, as far as the actual communication line is concerned, a line length, a line diameter and the combination thereof are different in each case and therefore the impedance of the communication line also differs from each other. As a result, the echo condition is also completely different for each line and problems can arise with the conventional switching exchanger where the echo still remains because it is difficult to cancel the echo in all lines.